TiC, TiCN, and TiN Supported Pt Electrocatalysts for CO and Methanol

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TiC, TiCN and TiN Supported Pt Electrocatalysts for CO and Methanol Oxidation in Acidic and Alkaline Media Maria Roca-Ayats, Gonzalo García, Jose Luis Galante, Miguel Antonio Peña, and Maria Victoria Martínez-Huerta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp407260v • Publication Date (Web): 11 Sep 2013 Downloaded from http://pubs.acs.org on September 19, 2013

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The Journal of Physical Chemistry

TiC, TiCN and TiN Supported Pt Electrocatalysts for CO and Methanol Oxidation in Acidic and Alkaline Media M. Roca-Ayats, G. García*, J.L. Galante, Miguel A. Peña and M.V. MartínezHuerta* Instituto de Catálisis y Petroleoquímica, CSIC. c/ Marie Curie 2. 28049. Madrid, Spain *Corresponding authors: María Victoria Martínez Huerta. Fax: +34 5854760; Tel: +34 5854787. E-mail: [email protected] Gonzalo García. Fax: +34 5854760; Tel: +34 5854792. E-mail: [email protected],

ACS Paragon Plus Environment


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Abstract TiC, TiCN and TiN supported Pt nanoparticles have been investigated as anode electrocatalytic materials for direct methanol fuel cells (DMFCs). The catalysts were studied in acidic and alkaline media and compared with platinum supported on carbon black. CO and methanol oxidation were studied by voltammetry and chronoamperometry techniques. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis were employed to characterize the novel catalysts. Results show that carbon supported Pt catalyst is mainly formed by nanoparticles with long (111) domains and those catalysts with a titanium-based support present a huge amount of defect sites with diverse symmetries. Additionally to geometric factors, an electronic effect by the Ti-based support leads to a considerably enhance of CO electrooxidation with respect to carbon supported catalyst, which is of special relevance in alkaline media. However, no such improvement is observed during the methanol oxidation reaction on Tibased catalysts at high pH.

Keywords Fuel cells, CO oxidation, methanol oxidation, DMFC, Titanium-based supports, platinum, alkaline media, acidic media, electrocatalysts.


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1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are considered to be a promising option for solving several energy problems. Hydrogen PEMFCs have been well developed but one of their main drawbacks is the poisoning of the catalyst by CO traces in the hydrogen obtained by reforming. Direct methanol fuel cell (DMFC) has some important advantages. Methanol is cheap and liquid at room temperature and as consequence is easy to manipulate and transport. Therefore, DMFC appears as a promising technology to be introduced in the energetic market, e.g. it can be easily introduced into the distribution network. However, one of the main problems is still the development of anodes with good performance. Nowadays, the anode contains a high loading of noble metals (e.g. Pt and Ru) and its kinetic is quite sluggish. Therefore, one of the principal tasks of low temperature fuel cell catalytic technology is developing cheap materials with great catalytic activity and high durability in the highly corrosive medium

1-4

. The use of alkaline membrane fuel cells (AMFCs) operating with

solid membranes is a good alternative to the PEMFC and DMFC. An enhancement of alcohol and CO oxidation with the rise of the alkalinity has been observed2,5-10. The electrooxidation of methanol is a complex reaction, involving different steps and intermediates

3-7,11-13

. The main catalytic problem is the poisoning of the Pt-

based electrodes by CO, which is an intermediate during the methanol oxidation reaction. Therefore, the study of the electrooxidation of CO becomes necessary, not only for a practical but also for fundamental science9,10,14-17. It should be also considered that the reaction is dependent on the atomic surface orientation of the catalyst as well as on the electrolyte (counter-ion), applied



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potential, methanol coverage, electrode material and pH. An alkaline electrolyte has some advantages such as being less corrosive or that all pH dependent reactions take place at much lower overpotentials respect to normal hydrogen electrode (NHE), which involves a decreasing in the adsorption strength of anions. This is especially important because anions such as sulphate inhibit many electrochemical processes. The major disadvantage of alkaline electrolytes is the formation of carbonate in air presence, which adsorbs irreversibly on the electrode, reducing the amount of available catalytic sites 8. One way to enhance the catalytic activity of platinum is through the catalyst support. The nature of the support can modify the size, shape and distribution of the platinum nanoparticles, leading to a modification of the number and nature of active sites, which could modify considerably the catalytic activity of the catalyst. The support may provide synergetic effects, changing the electronic properties of the active sites, which affects the adsorption strength of molecules in the surface and consequently the reactivity18-23. Furthermore, catalyst support is really important for the catalytic stability3,20,21. The most often used support for this kind of catalyst is carbon black, which is a really cheap material. However, carbon suffers corrosion under the extremely oxidative conditions in which fuel cells work, which leads to a low durability of the electrocatalysts and lost or agglomeration of platinum18,24. The latter is even more drastic during the starstop cycles3. This last issue motivated us to study novel catalyst supports with high corrosion resistance. Titanium carbide, nitride and carbonitride are reported to have high conductivity as well as high resistance towards corrosion25, and appear as a good candidate to modify the electronic structure of noble metals interacting


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with them. In this sense, theoretical calculations predict a strong interaction between TiC and Pt nanoclusters, with an important electron density transfer from the support to Pt, which leads to an enhancement of the adsorption strength of small molecules on Pt surface26. A previous study by Ou et al. revealed a good performance of TiC-supported Pt catalysts during methanol electro-oxidation in acidic media27. TiN-supported Pt electrodes have also been used for the methanol oxidation28 and oxygen reduction reactions29 in alkaline and acidic media, respectively. These studies reported high catalytic activity for both reactions. TiCN material, as far as we know, hasn’t been reported as electrocatalysts support until now. The objective of the present study is the systematic research of the support effect on Pt electrocatalysts for CO and methanol oxidation in acidic and alkaline media. TiC, TiN and TiCN have been studied and evaluated as titanium-based supports materials for low temperature fuel cells, and the results have been compared with catalyst supported on carbon black.

2. EXPERIMENTAL SECTION 2.1. Catalysts Synthesis. Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN materials were obtained following the ethylene glycol (EG) method30. Briefly, an appropriate amount of PtCl4 (99.99%, Alfa Aesar) to obtain nominal metal loading of 20 wt. % was dissolved in EG, and added into a previously made suspension of the support in EG. Carbon black (Vulcan XC-72R; BET surface area: 230 m2·g-1) from Cabot Corporation, and Ti-based materials (TiC, BET surface area: 23 m2·g-1; TiCN (7:3), BET surface area: 22 m2·g-1 and TiN, BET surface area: 2 m2/g) from Sigma-Aldrich were used as catalyst supports. The synthesis was carried out under N2 flow at



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160ºC at pH = 11 during 3 hours. After that, the pH was adjusted to 2 and the catalysts were first washed with acetone and later copiously with water. Then, the sample was dried in a furnace at 80ºC. Subsequently, a thermal treatment at 250ºC under He flow during 1h was carried out.

2.2 Physicochemical Characterization. Pt loading was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Perkin–Elmer Optima 3300 DV spectrometer. Xray diffraction (XRD) powder patterns were obtained on a PANalytical X’Pert Pro X-ray diffractometer using a Cu Kα source. The diffraction profiles of the samples were recorded within Braggs angles, ranging from 2º to 90º, at a scanning rate of 0.04º·s-1. Particle size and morphology were evaluated from the transmission electron microscopy (TEM) images obtained in a JEM 2100F microscope operating at an accelerating voltage of 200 kV. X-ray Photoelectron Spectroscopy (XPS) data were obtained with a SPECS customized system for surface analysis equipped with a non-monochromatic X-ray source XR 50 and a hemispherical energy analyzer PHOIBOS 150. X-ray Mg Kα line (1253.6eV) was used as excitation (operating at 100 W/10 kV), and the highest transmission mode of the lenses was used for the detector. Powder samples were attached onto Cu foil and were placed first in the pre-treatment chamber at room temperature for 1 h before being transferred to the analysis chamber. The instrument operated typically at pressures ca. 8•10-9 mbar in the analysis chamber. The XPS data were signal averaged for an enough number of scans to get a good signal/noise ratio, at increments of 0.1 eV and fixed pass energy of 25 eV.. High-resolution spectra envelopes were obtained by curve fitting


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synthetic peak components using the software XPSpeak. The raw data were used with no preliminary smoothing. Symmetric Gaussian-Lorentzian product functions were used to approximate the line shapes of the fitting components. Binding energies were calibrated relative to the C 1s peak from the graphitic carbon at 284.6 eV.

2.3 Electrochemical Measurements. Experiments were carried out in a three-electrode cell at room temperature with an Autolab PGSTAT302N potentiostat-galvanostat. A carbon rod was used as counter electrode and a reversible hydrogen electrode (RHE) in the supporting electrolyte as reference. The working electrode was prepared drying 20 µL of the catalyst dispersion onto a glassy carbon electrode (0.28 cm2) under Ar atmosphere. The dispersion was prepared by mixing 2.0 mg catalyst, 15 µL Nafion (5%, Sigma-Aldrich) and 500 µL water (Milli-Q, Millipore). The electrolyte was 0.5 M H2SO4 (Merck, p.a.) or 0.1 M NaOH (99.99 %, Sigma Aldrich) solution. 2 M CH3OH (Scharlau, HPLC grade) solution in the electrolyte (acid or alkaline) was utilized to perform methanol oxidation experiments. N2 (99.99%, Air Liquide) was used to deoxygenate all solutions and CO (99.997%, Air Liquide) to dose CO. For alkaline measurements, a cell that allows the electrolyte exchange at controlled potentials was used. Therefore, the electrolyte was renewed after every measurement to avoid the carbonate formation. Activation of the electrode was performed by potential cycling solution between 0.05 and 0.9 V vs RHE at a scan rate of 0.1 V·s-1 until a stable voltammogram (blank) in the base electrolyte was obtained (~50 cycles). CO stripping experiments were achieved at 0.02 V·s-1 after bubbling CO through the



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cell for 10 min while keeping the electrode at 0.10 V, followed by N2 purging and electrolyte exchange to remove the excess CO. CO stripping voltammograms were recorded, by first scanning negatively until 0.05 V so that entire hydrogen region was probed, and then scanning positively up to 0.9 V. Methanol current transients were obtained by stepping the potential from 0.05 V to the final oxidation potential (0.55 < Ef < 0.65 V). All current densities were normalized with respect to the electroactive area, obtained from the oxidation of a CO monolayer.

3. RESULTS AND DISCUSSION 3.1 Physicochemical Characterization. The experimental Pt-loadings determined by ICP-OES are shown in Table 1. The amount of Pt that was actually incorporated into each sample is lower than the theoretically expected. Pt loss takes place along the washing step. Additionally, similar atomic ratio Pt/Ti was observed in all Ti-samples. The HRTEM images and the corresponding histograms in Fig. 1confirm the influence of the catalyst support respect the particle size, i.e. metal deposition and growth during the nanoparticle synthesis strongly depend on the material support. The average particle size of the carbon supported Pt (2.8 nm) is slightly lower than that observed for the catalysts supported on Ti-samples (3-4 nm) (Table 1). Also, it is observed a rise in the agglomeration degree when nitrogen is present in the catalyst support. Nevertheless, the lower specific surface area of the TiN has to be considered. X-ray diffractograms of supported catalysts are shown in Fig. 2. The first peak in the pattern of Pt/C catalyst, at the low 2θ range (24.9º), is associated with the


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carbon black support. The diffraction peaks of Pt appear at 2θ values that are close to that expected for fcc Pt, i.e., 39.6º (111), 46.1º (200), 67.2º (220) and 81.1º (311). The X-ray diffraction patterns of TiC (JPCDS 006-0614), TiCN (JPCDS 042-1489) and TiN (JPCDS 038-1420) supports show reflections that are characteristics of supports crystallizing in the cubic pattern. Photoelectron spectroscopy analysis was used in order to obtain further information on the catalysts surface including the oxidation state of the elements. This information must be interpreted taking into account its limitations since the catalysts oxidation state probably changes during the electrochemical process. The XPS analysis of Pt 4f and Ti 2p are shown in Fig. 3. The Pt 4f signal (Fig. 3A) could be resolved into three doublets in all electrocatalysts. The most intense Pt 4f7/2 component at lower binding energy is attributed to metallic Pt. The second doublet can be assigned to Pt (II) in PtO and Pt(OH)2 like species, while the third doublet at higher binding energy corresponds to the higher oxidation state of Pt(IV). According to Table 2, there is a peak shift of metallic Pt to lower binding energy for titanium-based supports, and especially for nitrogen-containing supports. The latter indicates that a charge transfer from the Ti-based supports to platinum is taking place. Fig. 3B shows the Ti 2p spectra of Pt/TiC, Pt/TiCN and Pt/TiN samples. The binding energies are given in Table 2. Pt/TiC and Pt/TiCN consist of two overlapping doublets, while Pt/TiN shows only one doublet. The doublet peak at lower binding energies (Ti 2p3/2 ca. 254 eV) corresponds to titanium bounded to C or/and N, and the other doublet peak (Ti 2p3/2 ca. 258 eV) corresponds to titanium oxide31. The samples are covered by a small oxide layer of TiO2, which contribution is in all three



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catalysts higher than that of reduced titanium. The surface oxide on top of TiN covers the signals of reduced Ti.

3.2 Electrochemical Measurements. Fig. 4 and 5 show the CO stripping and the subsequent voltammograms in acidic and alkaline media, respectively. It is observed similar voltammetric profiles between the blank and subsequent voltammograms, indicating a low surface modification with the excursion towards anodic potentials. On the other hand, the first anodic scan develops several anodic peaks at different potential regions, which are associated to the CO oxidation to form CO2 at different catalytic sites. It is noticeable that these anodic peaks depend on both, pH and catalyst nature. In this sense, it is observable a strong influence of the catalyst support on the catalytic activity towards CO oxidation. Table 3 shows the onset and peak potentials values for the CO oxidation on all catalysts in acidic and alkaline media. In general, there is a decrease of the onset and peak potentials of the titanium-based support catalysts, with respect to carbon support catalyst. In addition, catalysts supported on titanium-based materials present more CO oxidation peaks than the Pt/C ones, both in acidic and alkaline media. In alkaline media, there is a shift toward more negative peak potential values and an increase on the number of oxidation peaks for all catalyst in comparison to acidic media. Fig. 6 and 7 show the potentiodynamic oxidation of methanol in acidic and alkaline media, respectively. It is observed an increment of the anodic current, which is associated to methanol oxidation, at potentials higher than 0.5 V for all catalysts in both media. A careful inspection of Fig. 6 and 7 bring out some


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differences during the methanol oxidation reaction between both media. Firstly, it is observed higher anodic currents during methanol oxidation on Ti-based catalysts in alkaline media. However, the anodic current decreases faster for all catalysts during the backward scan at high pH, which indicates a faster catalyst deactivation in alkaline media. Additionally, the onset potential for methanol oxidation in both media decreases in the following way: Pt/C > Pt/TiCN ≈ Pt/TiN > Pt/TiC. Fig. 8 and 9 show the current transients obtained at a final potential of 0.625 V for all catalysts in acidic and alkaline media, respectively. An important decrease of the current density along the time is observed for those catalysts supported on titanium-based materials, especially in alkaline media. On the other hand, the anodic current delivered in acidic media decreases in the subsequent way: Pt/TiN > Pt/C > Pt/TiCN > Pt/TiC. However, Pt/C presents the best performance towards methanol oxidation in alkaline media followed by Pt/TiN, Pt/TiCN and Pt/TiC catalysts. Tafel plots (Fig. 10) were obtained from the current transients of methanol oxidation (Fig. 8 and 9) at different final potentials (0.55 < E < 0.65 V) in acid and alkaline media. Similar slope values were obtained for all catalyst in acidic media. On the other hand, the delivered current densities strongly depend on the catalyst support at high pH. Furthermore, all catalysts present Tafel slopes much higher in alkaline media compared with those obtained in acidic media.

3.3 Platinum Surface Structure. Adsorption and oxidation reactions of CO and methanol are known to be really dependent on the surface structure of platinum catalysts. Therefore,



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particle size, structural effects and metal-support interactions are of crucial importance in the electrocatalytic activity. It is well known that the hydrogen region of the Pt blank voltammetry is sensitive to the surface structure, and this sensitivity can be employed to obtain a first idea of the nanoparticle surface structure9. Previous studies of Pt single crystals and well defined nanoparticles in sulphuric media have demonstrated that the reversible peaks observed for the blanks voltammograms at ca. 0.12 and 0.25 V are associated to the adsorption state on sites with (110) and (100) orientation. The same hold for the reversible peaks at ca. 0.26 and 0.38 V developed in alkaline medium. Hydrogen adsorption on (100) terraces (long domains) in sulphuric media is associated with a broad reversible peak at ca. 0.37 V, which is slightly observed for Pt/C catalyst (Fig. 4). Additionally, the reversible peaks at 0.55 and 0.80 V in acid and alkaline media, respectively, are related to anion (sulphate and hydroxide) adsorption on (111) sites with long domains9,32. According to these studies, we can establish that our catalysts have surface sites with (111), (110) and (100) orientation, but only Pt/C present long domains with (100) orientation, as well as, Pt/TiN is the only electrode in absence of long (111) domains. In the next section we are going to confirm the stated before.

3.4 CO oxidation on Pt sites. Regarding to CO oxidation reaction, defect sites (e.g. steps and kinks) are known to be more adsorptive and more reactive than flat surfaces. Using stepped Pt electrodes in acidic media, it has been concluded that all CO species, both those initially adsorbed at or near the step and those initially adsorbed on the terrace, diffuse and react to CO2 at the step sites, developing


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just only one anodic peak. CO oxidation on terrace is essentially negligible9,10. On the other hand, the catalytic activity for CO oxidation on Pt in alkaline solution is higher than in acidic solution. During the CO oxidation on Pt stepped single crystals in alkaline media, several anodic peaks were resolved and it was possible to deduce a clear correlation between them and the specific catalytic site16. The latter was possible due to a limited CO diffusion produced by adsorbed carbonate that blocks the reaction sites10. After that, it is possible to analyze the CO stripping voltammograms obtained at the present catalysts (Fig 4 and 5). In acidic media, all catalysts develop a main CO oxidation peak. However, some differences can be found. For example, Pt/C presents only a symmetric peak at 0.83 V, while the other catalysts exhibit an asymmetrical peak with several humps. These small anodic contributions are linked to CO oxidation on catalytic sites of diverse nature. Catalytic sites of diverse activity justify the absence of only one symmetric main peak in conjunction with a CO diffusion impediment, which can be occasioned by adsorbed sulfate on special sites (e.g. edge) or by some geometric factor (e.g. particle agglomeration) that impedes the free CO diffusion towards the most catalytic sites. Additionally, an electronic charge transfer by the catalyst support may produce higher CO binding energy and consequently slower CO diffusion16. Consequently, multiple anodic peaks associated to CO oxidation on different sites are observed during the CO stripping voltammogram15. Summarizing, it seems that Pt/C presents highly coordinated surface domains with (100) and (111) symmetries that are easily connected, while Ti-based catalysts have higher density of defect sites (lower-coordinated sites) and agglomeration (grain boundary sites) with higher activity than those observed at



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Pt/C material though the CO diffusion between these sites is not so straightforward. On the other hand, a close inspection of Fig. 4 shows an increment of the catalytic activity in acidic media in the following way: Pt/C < Pt/TiC < Pt/TiCN < Pt/TiN. In fact, the anodic contribution at low over-potentials (0.77 V) is absent at the Pt/C and increases from Pt/TiC to Pt/TiCN. Finally, Pt/TiN presents a slow CO oxidation at very low over-potential, which starts close to 0.28 V. Then, the anodic current rises at 0.56 V and two main contributions are observed at 0.72 and 0.77 V. In this context, the higher catalytic activity observed at Ti-based catalysts can be the consequence of multiple factors such as high density of low coordinated sites (i.e. defect sites), high degree of nanoparticles agglomeration or some promoter/synergetic effect of the catalyst support. A close inspection of TEM images suggests a correlation between particle size, agglomeration and catalytic activity. All Ti-based catalysts, and especially Pt/TiN, present low BET surface areas that lead to higher Pt particle size with higher agglomeration degree than Pt/C catalyst. In this sense, it is well recognized that sites at the emergence of the grain boundary regions play an important role on the catalytic activity33. This interaction between inter-particles makes the catalyst more reactive, which decreases the onset potential for the CO oxidation reaction. Therefore, the high agglomeration degree of Ti-based catalysts is an important factor for the enhanced activity observed. Nevertheless, electronic effects due to the support cannot be discarded. XPS results show a displacement of the platinum 4f peaks to lower energies for Ti-based catalysts, which clearly indicate the existence of an electronic effect. Accordingly, a negative charge


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transfer from the support to platinum nanoparticles may take place. The latter influences the adsorption of species (e.g. CO, OH in the bifuntional mechanism16,21), and in turn, the catalytic reactivity. Additionally to the described before, TiO2, which is present in the catalyst surface may produce a charge transfer in addition to deliver oxygenated species. The electronic effect on the Pt particles may produce a change in the adsorption strength of species, while the oxygenated species on the TiO2 surface may promote the CO oxidation reaction. In this context, it is well known the beneficial effect of transition and rare metal oxides on Pt-group metals during the CO oxidation reaction21,34. The increase in the catalytic activity was ascribed to the development of electron enriched Pt sites by a direct electronic effect promoted by the interaction of the noble metal and the transition and/or rare metal oxides21,34. In alkaline media, a higher catalytic activity is observed for all electrodes. It may be caused by different factors, being one of the most important the higher electronegativity of the catalyst surface. The latter decreases the adsorption strength of spectator species (e.g. anions such as sulfate), may increase the concentration of reactants (OH) and vary the adsorption strength of CO10,13,16. On the other hand, several anodic peaks during the CO stripping are observed. The anodic peak at 0.8 V corresponds to the CO oxidation on (111) sites with long domains, which appears at exactly the same potential as OH adsorption in the blank profile9,10,14,16,35. It is observed a current charge decreases of this peak in the following way: Pt/C > Pt/TiC > Pt/TiCN ≈ Pt/TiN. The latter suggests the same trend for the length domains of (111) sites, i.e. Pt/TiCN and Pt/TiN catalysts present short and small amount of well-ordered (111) domains and



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therefore all CO can diffuse and react on sites with higher catalytic activity. On the other hand, Pt/C and Pt/TiC comprise quite-long (111) domains and the produced carbonate may block the CO diffusion. Therefore, OH formed on (111) terrace sites at high over-potentials can only remove CO adsorbed on these high-coordinated sites. Following with the surface description, the anodic peak at ca. 0.59 V (clearly distinguished at Pt/TiCN and Pt/TiN and observed as a shoulder at Pt/TiC) corresponds to the CO oxidation on sites with (110) orientation

10,16,36

, while the

thin anodic peak at ca. 0.66 V is typical for the CO oxidation on sites with (100) orientation14,36. However, the intensity (i.e. charge density) of this peak does not correspond to the amount of (100) sites that should be expected from the hydrogen adsorption region in the blank profiles. A plausible explanation should be the presence of (111) domains with different length. CO adsorbed on long (111) terraces gives a large peak at 0.8 V since carbonate adsorbs strongly, blocking completely the diffusion of CO to more active sites, while in small (111) domains, the blocking effect of carbonate is not complete and CO is able to arrive and react at more active sites (e.g. (100) sites), contributing in the peak developed at 0.66 V. Nevertheless, this peak is broader for Pt/C. A probable explanation could be the presence of (100) sites with long domains, which can be only guessed in the blank voltammogram of Pt/C due to the capacitive current of the carbon support. Therefore, the CO oxidation peak at 0.66V developed at Pt/C could be understood as the convolution of those different contributions, i.e. (100) sites with long and short domains in addition to the peak associated to (111) sites. Finally, the anodic pre-wave with onset at ca. 0.3 V observed at Pt/TiCN and Pt/TiN catalysts can be associated to CO oxidation on


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low-coordinated (e.g. kink and step defect sites) or grain boundary sites10,16,37. The last option seems to be the operative since the particle agglomeration degree increases with nitrogen content in the catalyst support. However, charge interaction between support and catalyst has to be taken into consideration. After that, it is possible to have a whole picture of the different surface structure of the catalysts. The amount of well-ordered (111) domains decreases in the following way: Pt/C > Pt/TiC > Pt/TiCN > Pt/TiN, while the opposite trend holds for the increment of defect sites, i.e. low-coordinated sites with diverse symmetries). Additionally, Pt/C seems to be the only catalyst that contains long domains of (100) sites. As summary, the higher catalytic activity of titanium-based catalysts should be explained in terms of different types and amount of catalytic sites, particle agglomeration and charge transfer from the support towards the metal particles. Additionally, TiO2 presence in the catalyst surface may cause a synergetic effect on this reaction.

3.5 Methanol oxidation. Figures 6 and 7 depict the voltammetric profiles of the methanol oxidation on Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in acidic and alkaline media. It is observed that Pt/TiN and Pt/TiC present the best performance in acidic and alkaline media, respectively. However, a faster deactivation during the backward scan in alkaline media was observed, though the delivered current densities are higher. In agreement with the potentiodynamic experiments, the current transients recorded at high pH show faster deactivation compared with that achieved in acidic media (Fig. 8 and 9). Also, it is observed that the



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catalytic activity change drastically during the potentiostatic experiments in alkaline medium. In fact, figure 10 depicts that Pt/C presents the best performance in alkaline media in all potential range studied. Independently of the medium, it is observed an increment in the catalytic activity at Ti-based catalysts with the introduction of nitrogen in the catalyst support. However, Ti-based catalysts develop a constant and fast current decay along the time in alkaline solution. On the other hand, similar Tafel slopes were obtained for all Ti-based catalysts in both media, with higher slopes in alkaline media (Fig. 10). This suggests similar reaction mechanism and consequently similar deactivation process. On the other hand, Pt/C electrode presents a higher increment of Tafel slopes with the rise of alkalinity, which indicates a different reaction mechanism or a higher number of parallel interactions by intermediate and/or product of the reaction when pH is changed8,35. Methanol electrooxidation reaction has been widely studied in acidic media and less in alkaline solution1-8,11-13,21-23,28,35. It is well known that methanol electrooxidation undertakes parallel and by-side reactions. Firstly, methanol may adsorb on a suitable surface and after that it may suffer diverse dehydrogenation steps. In this sense, methanol could be dehydrogenated to form adsorbed CO during the recognized indirect pathway21,38 [1]. Then, CO can be fully oxidized to CO2 in acidic media or to carbonate at high pH. On the other hand, the direct pathway produces soluble species such as formaldehyde and/or formic acid21,38. These species may re-adsorb and be fully oxidized to CO2 or carbonate and/or formate depending on the pH of the solution. In this context, the main inconvenient of the direct pathway is the diffusion of intermediates or by-side products toward the electrolyte that decreases the


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conversion efficiency of the process. On the other hand, CO is a very wellknown catalytic poison and is the principal problem during the indirect pathway. Additionally, it is recognized that (100) and (110) domains are more active than (111) ones for the methanol electrooxidation reaction in acidic media13. According to the fact that our titanium-based catalysts showed high catalytic activity towards CO electrooxidation, it was expected good performance also during the methanol electrooxidation. However that it is not observed in the present work. Pt/TiN is the only catalyst that presents similar performance than Pt/C in acidic media, whereas all Ti-based catalysts show lower activity than the Pt/C ones in alkaline solution. These results suggest that is not only necessary a high density of (100) and (110) sites for an enhanced CO tolerance, but also a suitable surface for methanol adsorption. The last seems to be the principal cause for the poor methanol oxidation reaction on Ti-based catalysts. In fact, the high amount of oxygenated species on the surface (titanium oxide), which is increased at high pH, appears as the main responsible for the low methanol adsorption and consequently low catalytic activity towards methanol oxidation. The last suggestion is in total agreement with the well-known catalytic activity decrease on platinum oxide surface3,11. Moreover, the rise of the Tafel slope at Pt/C with the alkalinity is well documented by studies of Spendelow et al. in which obtained values close to 200 mV dec-1 on Pt (111) single crystal electrodes in alkaline media. They suggested an accumulation of intermediates in the surface, which decreases the methanol adsorption rate35. In this context, the increment of Tafel slopes with the rise of pH for Ti-based catalysts is lower for for the Pt/C catalyst. Therefore, in catalysts with Ti-containing supports, lateral repulsion by intermediates seems to be less important and methanol



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dissociative adsorption appears again as responsible for the low methanol oxidation rate at high pH. Consequently, at electrochemical interfaces, not only the properties of the nanostructured catalyst surface influence the kinetics and catalysis of the electrochemical reaction, but also the interaction with the Ti-based support and the electrolyte components, primarily the solvent and the ions, which in turn will impact on the electrocatalytic activity. This leads to a complex and dynamic interplay between catalyst surface, support, reactants, intermediates, and electrolyte components.

4. CONCLUSIONS Novel Pt-based catalysts supported on TiC, TiN and TiCN were evaluated for the methanol and CO electrooxidation in acidic and alkaline media and compared with platinum supported on carbon Vulcan. It was noticed that titanium-based supports strongly affect the properties of platinum nanoparticles during the synthesis. In this sense, Pt nanoparticles with different particle size, agglomeration degree and number of surface defects were created. Additionally, it was observed an increment of defect sites with the concentration of nitrogen in the catalyst support. It was observed an important enhancement of the catalytic activity for CO oxidation reactions when TiC, TiN and TiCN were used as catalyst support. The improved tolerance towards CO was ascribed to facile oxygenated species formation on Ti-based catalysts (higher density of catalytic sites) and to pure electronic effects (influence of the catalyst support on the electronic state of the


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active phase), and both effects were enhanced with the increment of nitrogen in the catalyst support. The performance towards methanol electro-oxidation was also influenced by the catalyst support. Nitrogen presence in the catalyst support enhances the catalytic activity towards methanol oxidation. However, this reaction decreases at Ti-based catalysts in alkaline media. It is suggested the methanol dissociative adsorption as responsible for the methanol oxidation rate on Pt/Ti-based materials.

Acknowledgements This work has been supported by the Spanish Science and Innovation Ministry under projects ENE2010-15381 and CTQ2011-28913CO2-O2. MR and GG acknowledge to the FPU-2012 program and the European Social Fund and JAE Program (CSIC) for financial support.

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Table 1. Metal loading (from ICP-OES), and average particle size (from TEM) of catalysts, and BET area of supports Catalyst

Pt loading (wt%)

Pt/Ti

Average

BET area

atomic ratio

particle size (nm)

(m2·g-1)

2.8±1.0

230

Pt/C

17

Pt/TiC

14

0.06

3.4±1.0

23

Pt/TiCN

16

0.07

4.5±1.5

22

Pt/TiN

14

0.07

4.3±1.1

2

Table 2. XPS results for the electrocatalysts Catalyst

Pt species BE of

FWHM (eV)

Ti species BE of

Pt 4f7/2 (eV) Pt/C

Pt/TiC

Pt/TiCN

Pt/TiN

FWHM (eV)

Ti 2p3/2 (eV)

atomic ratio

Pt(0)

71.1 (68)*

1.6

Pt(II)

72.7 (21)

1.6

Pt(IV)

74.4 (11)

1.6

Pt(0)

70.6 (73)

1.6

TiC

454.6 (18)

1.8

Pt(II)

72.0 (22)

1.8

TiO2

457.9 (82)

1.6

Pt(IV)

73.5 (5)

1.8

Pt(0)

70.4 (85)

1.8

TiCN

454.5 (28)

1.8

Pt(II)

72.1 (11)

1.8

TiO2

457.6 (72)

1.8

Pt(IV)

73.3 (4)

1.8

Pt(0)

70.1 (58)

1.7

TiO2

457.3 (100)

2.1

Pt(II)

72.0 (27)

1.8

Pt(IV)

73.8 (15)

1.8

*Approximate oxidation state distribution expressed in %


Pt/Ti

0.37

0.33

2.99


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Table 3. Onset and peak potentials values for CO oxidation on Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in acidic and alkaline media. Pre-peak

Main peak

Onset (V)

Onset (V)

Acid / Alkaline

Acid / Alkaline

Pt/C

---

0.70 / 0.53

0.83 / 0.69, 0.80

Pt/TiC

---

0.57 / 049

0.77, 0.82 / 0.67, 0.79

Pt/TiCN

--- / 0.32

0.57 / 0.53

0.77, 0.84 / 0.60, 0.67, 0.81

Pt/TiN

0.28 / 0.28

0.56 / 0.50

0.72, 0.77 / 0.57, 0.66, 0.80

Main peaks (V) Catalyst

Acid / Alkaline


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Figure captions Fig. 1: TEM images and distribution histograms of Pt/C (A), Pt/TiC (B), Pt/TiCN (C) and Pt/TiN (D). Fig. 2: XRD diffractograms obtained at Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts. Fig. 3: Pt 4f (A) and Ti 2p (B) XPS spectra of Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts. Fig. 4: CO stripping voltammograms recorded at Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.5 M H2SO4. Ead = 0.1 V, scan rate = 0.02 V s-1. Fig. 5: CO stripping voltammograms recorded at Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.1 M NaOH. Ead = 0.1 V, scan rate = 0.02 V s-1. Fig. 6: Methanol oxidation recorded at Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.5 M H2SO4. Ead = 0.1 V, scan rate = 0.02 V s-1. Fig. 7: Methanol oxidation recorded at Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.1 M NaOH. Ead = 0.1 V, scan rate = 0.02 V s-1. Fig. 8: Current transients for methanol oxidation on Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.5 M H2SO4. Ef = 0.625 V. Fig. 9: Current transients for methanol oxidation on Pt/C, Pt/TiC, Pt/TiCN and Pt/TiN catalysts in 0.1 M NaOH. Ef = 0.625 V. Fig. 10: Tafel plots of the chronoamperometric transients on the four catalysts. Top panel: 2M CH3OH + 0.5 M H2SO4; Bottom panel: 2M CH3OH + 0.1 M NaOH.



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Fig. 1


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Fig. 2



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A

B

Fig. 3


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Fig. 4



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100 Lowcoordi 110 nation sites

Fig. 5


111

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Fig. 6



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Fig. 7


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Fig. 8



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Fig. 9


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Fig. 10



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Table of Contents Graphic Pt (0)

Pt/C

Pt/C

Pt(II)

-2

50Acm

Pt (IV)

Pt/TiC

Pt/TiC

Pt/TiCN

Pt/TiCN

Pt/TiN

Pt/TiN

80 78 76 74 72 70 68

BindingEnergy / eV Binding Energy/eV

0.2

0.4

0.6

0.8

Potential //VVvsvs RHE Potential RHE


Page 38 of 38

TiC, TiCN, and TiN Supported Pt Electrocatalysts for CO and Methanol

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